Implantable wireless sensor

ABSTRACT

The progress of a endovascular aneurysm repair can be monitored by inserting a pressure transducer sensor using a catheter into the sac during endovascular aneurysm repair and then using a small, hand-held read out device to measure pressure easily, safely, inexpensively and accurately. In one aspect a sensor is introduced into the body by the steps of loading the sensor into a catheter, and deploying into the aneurysm sac. This invention also has other applications for measuring physical properties in patients or in other sites.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon co-pending, commonly assigned U.S.provisional patent application Ser. No. 60/503,745, filed Sep. 16, 2003,incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The application is directed to an implantable wireless sensor. Moreparticularly, this invention is directed to a wireless, unpowered,micromechanical sensor that can be delivered using endovasculartechniques, to measure a corporeal parameter such as pressure ortemperature.

BACKGROUND OF THE INVENTION

Abdominal aortic aneurysms represent a dilatation and weakening of theabdominal aorta which can lead to aortic rupture and sudden death.Previously, the medical treatment of abdominal aortic aneurysms requiredcomplicated surgery with an associated high risk of injury to thepatient. More recently, endografts (combining stents and grafts into asingle device) have been developed that can be inserted through smallincisions in the groin. Once in place, these endografts seal off theweakened section of the aorta. The aneurysms can then heal, eliminatingthe risk of sudden rupture. This less invasive form of treatment forabdominal aortic aneurysms has rapidly become the standard of care forthis disease. An example of an endograft device is disclosed inKornberg, U.S. Pat. No. 4,617,932.

A significant problem with endografts is that, due to inadequate sealingof the graft with the aorta, leaks can develop that allow blood tocontinue to fill the aneurysmal sac. Left undiscovered, the sac willcontinue to expand and potentially rupture. To address this situation,patients who have received endograft treatment for their abdominalaortic aneurysms are subjected to complex procedures that rely oninjection of contrast agents to visualize the interior of the aneurysmsac. These procedures are expensive, not sensitive, and painful. Inaddition, they subject the patient to additional risk of injury. See,for example, Baum R A et al., “Aneurysm sac pressure measurements afterendovascular repair of abdominal aortic aneurysms”, The Journal ofVascular Surgery, January 2001, and Schurink G W et al., “Endoleakageafter stent-graft treatment of abdominal aneurysm: implications onpressure and imaging—an in vitro study”, The Journal of VascularSurgery, August 1998. These articles provide further confirmation of theproblem of endograft leakage and the value of intra-sac pressuremeasurements for monitoring of this condition.

Thus, there is a need for a method of monitor the pressure within ananeurysm sac that has undergone repair by implantation of an endograftto be able to identify the potential presence of endoleaks. Furthermore,this method should be accurate, reliable, safe, simple to use,inexpensive to manufacture, convenient to implant and comfortable to thepatient.

An ideal method of accomplishing all of the above objectives would be toplace a device capable of measuring pressure within the aneurysm sac atthe time of endograft insertion. By utilizing an external device todisplay the pressure being measured by the sensor, the physician willobtain an immediate assessment of the success of the endograft at timeof the procedure, and outpatient follow-up visits will allow simplemonitoring of the success of the endograft implantation.

An example of an implantable pressure sensor designed to monitorpressure increases within an aneurysmal sac is shown in Van Bockel, U.S.Pat. No. 6,159,156. While some of the above objectives are accomplished,this device has multiple problems that would make its use impractical.For example, the sensor system disclosed in the Van Bockel patent relieson a mechanical sensing element that cannot be practically manufacturedin dimensions that would allow for endovascular introduction. Inaddition, this type of pressure sensor would be subject to many problemsin use that would limit its accuracy, stability and reliability. Oneexample would be the interconnection of transponder and sensor as taughtby Van Bockel, such interconnection being exposed to body fluids whichcould disrupt its function. This would impact the device's ability tomaintain accurate pressure reading over long periods of time. Afundamental problem with sensors is their tendency to drift over time. Asensor described in the Van Bockel patent would be subject to drift as aresult of its failure to seal the pressure sensing circuit from theexternal environment. Also, by failing to take advantage of specificapproaches to electronic component fabrication, allowing for extensiveminiaturization, the Van Bockel device requires a complex system foracquiring data from the sensor necessary for the physician to make anaccurate determination of intra-aneurysmal pressure.

OBJECTS OF THE INVENTION

It is an object of this invention to provide an implantable wirelesssensor.

It is also an object of this invention to provide a wireless, unpowered,micromechanical sensor that can be delivered endovascularly.

It is a further object of this invention to provide an implantable,wireless, unpowered sensor that can be delivered endovascularly tomeasure pressure and/or temperature.

It is a yet further object of this invention to provide a method ofpreparing a micromechanical implantable sensor.

It is a yet further object of this invention to provide amicromechanical sensor with a hermetically sealed, unbreached pressurereference for enhanced stability.

These and other objects of the invention will become more apparent fromthe discussion below.

SUMMARY OF THE INVENTION

The present invention comprises a device that can be implanted into thehuman body using non-surgical techniques to measure a corporealparameter such as pressure, temperature, or both. Specific targetlocations could include the interior of an abdominal aneurysm or achamber of the heart. This sensor is fabricated usingMicroElectroMechanical Systems (MEMS) technology, which allows thecreation of a device that is small, accurate, precise, durable, robust,biocompatible, radiopaque and insensitive to changes in body chemistry,biology or external pressure. This device will not require the use ofwires to relay pressure information externally nor need an internalpower supply to perform its function.

The MEMS approach to sensor design lends itself to the fabrication ofsmall sensors that can be formed using biocompatible materials assubstrate materials. The pressure sensor described above can beintroduced into the sac of an abdominal aneurysm at the time anendograft is deployed within the aorta by using standard endovascularcatheter techniques. Appropriately biocompatible coatings may be appliedto the surface of the sensor to prevent adhesion of biologicalsubstances or coagulated blood to the sensor that could interfere withits proper function.

In one embodiment of the invention an implantable wireless sensorcomprises two substrates, at least one of which has a recess. The sensorcomprises a self-contained resonant circuit comprising a capacitor andan inductor, where the circuit is variable in response to a physicalproperty, or changes in a physical property, of a patient. Thesubstrates are sealed together to form a hermetically scaled chamber,preferably one that is pressure sensitive.

In another embodiment of the invention one surface of each substratecomprises an inductor coil such as a wire spiral arranged in planarfashion. When the substrates are sealed together, the wire spirals arein planes parallel to each other.

In another embodiment of the invention each inductor coil is connectedby a wire to a capacitor plate arranged in the middle of the respectivecoil. The capacitor plates are substantially planar to the respectiveinductor coils and are substantially arranged parallel to each other.

In another embodiment of the invention the sensor may comprise ametallic basket arranged exterior to the substrates.

Delivery of the device of the invention to an aneurysm may beaccomplished as follows: Using the standard Seldinger technique, thephysician gains access to the patient's femoral artery and places avessel introducer with a hemostatic valve. Under direct fluoroscopicvisualization, a flexible guidewire is inserted through the introducercatheter and maneuvered such that its tip is stationed within the sac ofthe aortic aneurysm. A standard vessel introducer is inserted over theguidewire and through the introducer and advanced distally until its tipis within the aneurysmal sac. The inner dilator of the vessel introduceris removed and a sensor delivery vehicle is inserted the inner lumen ofintroducer. The delivery vehicle consists of a polymer support tube withtwo channels that run through its length, a metal or rigid sensorsupport capsule in which the sensor is placed and atraumatic tip.

The sensor is attached to a tethering system consisting of a hollow tubewith small diameter flexible wire disposed within. Near the terminal endof the hollow tube, a small break in the tube's surface is made. Theflexible tether wire emerges out of this break, is threaded through asmall hole in the rear section of the sensor, placed over the sensor,inserted through an identical hole in the forward segment of the sensorand re-inserted back into the hollow tube in a similar break in thetube's surface. In this configuration, the sensor remains secured to thetether wire after the delivery vehicle is removed from the patient.Following the insertion and deployment of the stent-graft, the sensor isdetached from the tether wire by simply retracting the wire from thehollow tube. Once the wire has been pulled through the two holes in thesensor, the sensor is released into the aneurysm sac and the wire andhollow tube are removed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an oblique perspective view of an embodiment of the invention;

FIG. 2 is a top, partly cross-sectional view of the embodiment of theinvention shown in FIG. 1;

FIG. 3 is a top, partly cross-sectional view of another embodiment ofthe invention;

FIG. 4 is an oblique, cross-sectional view of the embodiment of theinvention shown in FIG. 2;

FIG. 5 is an oblique, cross-sectional view of the embodiment of theinvention shown in FIG. 3;

FIG. 6 is a exposed cross-sectional view of the embodiment of theinvention shown in FIG. 5;

FIG. 7 shows part of the sensor tethering system;

FIG. 8 shows the further details of the tethering system;

FIGS. 9 to 12 show additional details of the tethering system;

FIGS. 13 to 15 show details of the delivery system;

FIGS. 16 to 26 show details of the manufacturing process used tofabricate the invention;

FIG. 27 represents an additional embodiment of the invention; and

FIG. 28 is a schematic of a control system.

DETAILED DESCRIPTION OF THE INVENTION

The invention can perhaps be better understood by referring to thedrawings. FIG. 1 is an oblique, perspective view of a sensor 2, anembodiment of the invention. Sensor 2 preferably has an outer coating ofbiocompatible silicone.

FIG. 2 is a top, partial cross-section of a schematic representation ofsensor 2 where a wire spiral inductor coil 4 is positioned in planarfashion in a substrate 6. Optionally sensor 2 may have recesses 8, eachwith a hole 10, to receive a tether wire (not shown here) for deliveryof the device into a human patient, as described below.

In the embodiment of the invention shown in FIG. 3, a wire 12 connectscoil 4 to a capacitor plate 14 positioned within coil 4.

FIG. 4 is a slightly oblique cross-section across its width of theembodiment of the invention shown in FIG. 2, where it can be seen thatsensor 2 is comprised of a lower substrate 20 and an upper substrate 22.Lower substrate 20 and upper substrate 22 are constructed from asuitable material, such as glass, fused silica, sapphire, quartz, orsilicon. Fused silica is the preferred material of construction. Lowersubstrate 20 has on its upper surface 24 an induction coil 26, and uppersubstrate 22 has a recess 28 with a surface 30 having an induction coil32 thereon. The top surface of upper substrate 22 forms a membrane 34capable of mechanically responding to changes in a patient's physicalproperty, such as pressure. The end 36 of sensor 2 has a notch or recess38.

In similar fashion, FIG. 5 is a slightly oblique cross-section acrossits width of the embodiment of the invention shown in FIG. 3. Theprimary difference between FIGS. 4 and 5 is the presence of uppercapacitor plate 42 and lower capacitor plate 44 on surfaces 24 and 30,respectively. In the embodiment of FIG. 4, the spiral coil 4 itself actsas the capacitive element of the LC circuit that describes the operationof the sensor.

FIG. 6 is a variation of FIG. 5 where the outline of upper substrate 22is shown but the details of lower substrate 20 can be seen more clearly,including individual coils of inductor coil 26. A wire 46 connects lowercapacitor plate 44 to induction coil 26.

The size of the sensors of the invention will vary according to factorssuch as the intended application, the delivery system, etc. The ovalsensors are intended to be from about 0.5 in. to about 1 in. in lengthand from about 0.1 in. to about 0.5 in. in width, with a thickness offrom about 0.05 in. to about 0.30 in.

As shown in FIGS. 4 and 5, upper substrate 22 can be significantlythinner than lower substrate 20. By way of example, upper substrate 22may be from about 100 to about 300 microns thick, whereas lowersubstrate 20 may be from about 500 to about 1500 microns thick. In analternate embodiment of the invention, both substrates may be of thesame thickness ranging from about 100 to about 1000 microns.

In the embodiment of the invention shown in FIG. 7, a sensor 50 isattached to a hollow tube 52 that has a flexible tip 54.

FIG. 8 shows the sensor 50 and specific features of the tetheringsystem, namely proximal holes 56 and distal holes 58 disposed in ahollow tube 52.

FIG. 9 shows a tether wire 60 that is attached to sensor 50 at sensorholes 62 and hollow tube holes 56 and 58, and a tether wire 60 ispositioned slidably within a hollow tube 52.

A better appreciation of certain aspects of the invention, especially ofa delivery system, can be obtained from FIG. 10 which shows a vesselintroducer 66 and the delivery system 68.

Further details of the delivery system are shown in FIG. 11. A doublelumen tube 70 has one channel that accepts a guidewire 72 and a secondchannel that accepts the sensor tether wire. The guidewire 72 can beadvanced through hub 74. A rigid delivery capsule 78 is disposed at thefar end of the delivery catheter and flexible tip 80 is connected to thecatheter via a hollow tube 81 extending through the delivery capsule 78.A sensor 82 is positioned inside a slot in the delivery capsule 78proximal to flexible tip 80.

FIG. 12 shows a lateral, cross-sectional view of this arrangement wherethe sensor 82 is inside the slot of delivery capsule 78 and the flexibletip 84 of the tether wire is disposed between the end of deliverycapsule 78 and flexible tip 80.

FIG. 13 shows delivery catheter 68 loaded into the previously placedvessel introducer 66 prior to introduction of the sensor into the body.

FIG. 14 shows that the sensor 82 on tether tube 52 has been advanced outof delivery capsule 78 and the delivery catheter has been removed.

In FIG. 15, the tether wire has been retracted into the hollow tethertube, releasing the sensor. The tether wire, tether tube and vesselintroducer 66 are then all removed.

The pressure sensor of the invention can be manufactured usingMicro-machining techniques that were developed for the integratedcircuit industry. An example of this type of sensor features aninductive-capacitive (LC) resonant circuit with a variable capacitor, asis described in Allen et al., U.S. Pat. Nos. 6,111,520 and 6,278,379,all of which are incorporated herein by reference. The sensor containstwo types of passive electrical components, namely, an inductor and acapacitor. The sensor is constructed so that the fluid pressure at thesensor's surface changes the distance between the capacitor'ssubstantially parallel plates and causes a variation of the sensor'scapacitance.

In a preferred embodiment the sensor of the invention is constructedthrough a series of steps that use standard MEMS manufacturingtechniques.

FIG. 16 shows the first step of this process in which a thin layer ofmetal (Protective mask) 90 is deposited onto the top and bottom surfaceof a fused silica wafer 92 (alternative materials would be glass,quartz, silicon or ceramic). Wafer diameters can range from about 3 toabout 6 in. Wafer thickness can range from about 100 to about 1500microns. A pattern mask is then created on one side of the wafer todefine the location of cavities that need to be etched into the surface.

FIG. 17 shows trenches or cavities 94 are etched into one surface of thewafer 92 to depths ranging from about 20 to about 200 microns. Thisetching is accomplished using any combination of standard wet and dryetching techniques (acid etch, plasma etch, reactive ion etching) thatare well known in the MEMS industry. The protective metal mask isremoved using standard metal etching techniques.

In FIG. 18, a thin metal seed layer 96 (typically chromium) is depositedon the etched side of the wafer using standard metal depositiontechniques such as sputtering, plating or metal evaporation.

In FIG. 19 a layer of photo-resistive material 98 is applied to theetched surface of the wafer using standard spin coating procedures.

FIG. 20 shows that a mask aligner and UV light 102 is used in aphotolithographic processes to transfer a pattern from a mask 104 to thephotoresist coating on the wafer.

In FIG. 21, the non-masked portions of the Photoresist are removedchemically creating a mold 106 of the desired coil pattern.

FIG. 22 shows copper 108 electroplated into the mold to the desiredheight, typically from about 5 to about 35 microns.

In FIG. 23, the Photoresist 110 and seed layer 112 are etched awayleaving the plated copper coils 114.

This process is then repeated with a second wafer.

In FIG. 24, the two processed wafers 118 and 120 are aligned such thatthe cavities 122 and 124 with plated coils are precisely orientated inover one another and temporarily bonded to each other.

FIGS. 25 and 26 show that by using a CO₂ laser 126 (or other appropriatelaser type), the individual sensors 130 are cut from the glass wafer.The laser cutting process results in a permanent, hermetic seal betweenthe two glass wafers. The laser energy is confined to a precise heateffect zone 128 in which the hermetic seal is created.

FIG. 27 represents an embodiment of the invention wherein a sensor 132attached to a delivery catheter 134 has a stabilizer or basket 136. Thestabilizer can be any appropriate device or structure that can befixedly attached to a sensor of the invention to assist the sensor inmaintaining position, location, and/or orientation after the sensor isdelivered to an intended site. The stabilizer can comprise anyappropriate physiologically acceptable rigid or slightly flexiblematerial, such as stainless steel, nitinol, or a radiopaque metal oralloy.

This sensor design provides many important benefits to sensorperformance. The hermetic seal created during the laser cutting process,coupled with the design feature that the conductor lines of the sensorare sealed within the hermetic cavity, allows the sensor to remainstable and drift free during long time exposures to body fluids. In thepast, this has been a significant issue to the development of sensorsdesigned for use in the human body. The manufacturing methodologydescribed above allows many variations of sensor geometry and electricalproperties. By varying the width of the coils, the number of turns andthe gap between the upper and lower coils the resonant frequency thatthe device operates at and the pressure sensitivity (i.e., the change infrequency as a result of membrane deflection) can be optimized fordifferent applications. In general, the design allows for a very smallgap between the coils (typically between about 3 and about 35 microns)that in turn provides a high degree of sensitivity while requiring onlya minute movement of the coils to sense pressure changes. This isimportant for long term durability, where large membrane deflectioncould result in mechanical fatigue of the pressure sensing element.

The thickness of the sensor used can also be varied to alter mechanicalproperties. Thicker wafers are more durable for manufacturing. Thinnersensors allow for creating of thin pressure sensitive membranes foradded sensitivity. In order to optimize both properties the sensors maybe manufactured using wafers of different thicknesses. For example, oneside of the sensor may be constructed from a sensor of approximatethickness of 200 microns. This wafer is manufactured using the stepsoutlined above. Following etching, the thickness of the pressuresensitive membrane (i.e., the bottom of the etched trench) is in therange of from about 85 to about 120 microns. The matching wafer is fromabout 500 to about 1000 microns thick. In this wafer, the trench etchingstep is eliminated and the coils are plated directly onto the flatsurface of the wafer extending above the wafer surface a height of fromabout 20 to about 40 microns. When aligned and bonded, the appropriategap between the top and bottom coils is created to allow operationpreferably in a frequency range of from 30 to 45 MHz and havesensitivity preferably in the range of from 5 to 15 kHz per millimeterof mercury. Due to the presence of the from about 500 to about 1000micron thick wafer, this sensor will have added durability forendovascular delivery and for use within the human body.

The sensor exhibits the electrical characteristics associated with astandard LC circuit. An LC circuit can be described as a closed loopwith two major elements, a capacitor and an inductor. If a current isinduced in the LC loop, the energy in the circuit is shared back andforth between the inductor and capacitor. The result is an energyoscillation that will vary at a specific frequency. This is termed theresonant frequency of the circuit and it can be easily calculated as itsvalue is dependent on the circuit's inductance and capacitance.Therefore, a change in capacitance will cause the frequency to shifthigher or lower depending upon the change in the value of capacitance.

As noted above, the capacitor in the assembled pressure sensor consistsof the two circular conductive segments separated by an air gap. If apressure force is exerted on these segments it will act to move the twoconductive segments closer together. This will have the effect ofreducing the air gap between them which will consequently change thecapacitance of the circuit. The result will be a shift in the circuit'sresonant frequency that will be in direct proportion to the forceapplied to the sensor's surface.

Because of the presence of the inductor, it is possible toelectromagnetically couple to the sensor and induce a current in thecircuit. This allows for wireless communication with the sensor and theability to operate it without the need for an internal source of energysuch as a battery. Thus, if the sensor is located within the sac of anaortic aneurysm, it will be possible to determine the pressure withinthe sac in a simple, non-invasive procedure by remotely interrogatingthe sensor, recording the resonant frequency and converting this valueto a pressure measurement. The readout device generates electromagneticenergy that penetrates through the body's tissues to the sensor'simplanted location. The sensor's electrical components absorb a fractionof the electromagnetic energy that is generated by the readout devicevia inductive coupling. This coupling induces a current in the sensor'scircuit that oscillates at the same frequency as the appliedelectromagnetic energy. Due to the nature of the sensor'selectromechanical system there exists a frequency of alternating currentat which the absorption of energy from the readout device is at amaximum. This frequency is a function of the capacitance of the device.Therefore, if the sensor's capacitance changes, so will the optimalfrequency at which it absorbs energy from the readout device. Since thesensor's capacitance is mechanically linked to the fluid pressure at thesensor's surface, a measurement of this frequency by the readout devicegives a relative measurement of the fluid pressure. If calibration ofthe device is performed, then an absolute measurement of pressure can bemade. See, for example, the extensive discussion in the Allen et al.patent, again incorporated herein by reference, as well as Gershenfeldet al., U.S. Pat. No. 6,025,725, incorporated herein by reference.Alternative readout schemes, such as phase-correlation approaches todetect the resonant frequency of the sensor, may also be employed.

The pressure sensor is made of completely passive components having noactive circuitry or power sources such as batteries. The pressure sensoris completely self-contained having no leads to connect to an externalcircuit or power source. Furthermore, these same manufacturingtechniques can be used to add additional sensing capabilities, such asthe ability to measure temperature by the addition of a resistor to thebasic LC circuit or by utilizing changes in the back pressure of gasintentionally sealed within the hermetic pressure reference to changethe diaphragm position and therefore the capacitance of the LC circuit.

It is within the scope of the invention that the frequency response tothe sensor will be in the range of from about 1 to about 200 MHz,preferably from about 1 to about 100 MHz, and more preferably from about2 to about 90 MHz, and even more preferably from about 30 to about 45MHz, with a Q factor of from about 5 to about 150, optimally from about5 to about 80, preferably from about 40 to about 100, more preferablyfrom about 50 to about 90.

In a further embodiment of the invention there is no directconductor-based electrical connection between the two sides of the LCcircuit. Referring again to the sensor described in the Allen et al.patents, the device is constructed using multiple layers upon lie thenecessary circuit elements. Disposed on the top and bottom layer aremetal patterns constructed using micro-machining techniques which definea top and bottom conductor and a spiral inductor coil. To provide for anelectrical contact between the top and bottom layers small vias or holesare cut through the middle layers. When the layers are assembled, ametal paste is forced into the small vias to create direct electricalconnections or conduits. However, experimentation has shown that due toadditional capacitance that is created between the top and bottominductor coils, a vialess operational LC circuit can be created. Thisabsence of via holes represents a significant improvement to the sensorin that it simplifies the manufacturing process and, more importantly,significantly increases the durability of the sensor making it moreappropriate for use inside the human body.

Further, the invention is not limited to the implantation of a singlesensor. Multiple pressure sensors may be introduced into the aneurysmspace, each being positioned at different locations. In this situation,each sensor may be designed with a unique signature (obtained bychanging the resonant frequency of the sensor), so that the pressuremeasurement derived from one sensor can be localized to its specificposition within the aneurysm.

A significant design factor that relates to the performance of thesensor and the operation of the system is the Quality factor (Q)associated with the sensor. The value of Q is one of the keydeterminates as to how far from the sensor the external read-outelectronics can be located while still maintaining effectivecommunication. Q is defined as a measure of the energy stored by thecircuit divided by the energy dissipated by the circuit. Thus, the lowerthe loss of energy, the higher the Q.

Additional increases in Q can be achieved by removing the centralcapacitive plate and using capacitive coupling between the copper coilsto act as the capacitor element.

In operation, energy transmitted from the external read-out electronicswill be stored in the LC circuit of the sensor. This stored energy willinduce a current in the LC loop which will cause the energy to be sharedback and forth between the inductor and capacitor. The result is anoscillation that will vary at the resonant frequency of the LC circuit.A portion of this ocscillating energy is then coupled back to thereceiving antenna of the read-out electronics. In high Q sensors, mostof the stored energy is available for transmission back to theelectronics, which allows the distance between the sensor and thereceiving antenna to be increased. Since the transmitted energy willdecay exponentially as it travels away from the sensor, the lower theenergy available to be transmitted, the faster it will decay below asignal strength that can be detected by the receiving antenna and thecloser the sensor needs to be situated relative to the receivingelectronics. In general then, the lower the Q, the greater the energyloss and the shorter the distance between sensor and receiving antennarequired for sensor detection.

The Q of the sensor will be dependent on multiple factors such as theshape, size, diameter, number of turns, spacing between turns andcross-sectional area of the inductor component. In addition, Q will begreatly affected by the materials used to construct the sensors.Specifically, materials with low loss tangents will provide the sensorwith higher Q factors.

The implantable sensor ascending to the invention is preferablyconstructed of various glasses or ceramics including but not limited tofused silica, quartz, pyrex and sintered zirconia, that provide therequired biocompatibility, hermeticity and processing capabilities.Preferably the materials result in a high Q factor. These materials areconsidered dielectrics, that is, they are poor conductors ofelectricity, but are efficient supporters of electrostatic orelectroquasiatatic fields. An important property of dielectric materialsis their ability to support such fields while dissipating minimalenergy. The lower the dielectric loss (the proportion of energy lost),the more effective the dielectric material in maintaining high Q. For alossy dielectric material, the loss is described by the property termed“loss tangent.” A large loss tangent reflects a high degree ofdielectric loss.

With regard to operation within the human body, there is a secondimportant issue related to Q, namely, that blood and body fluids areconductive mediums and are thus particularly lossy. The consequence ofthis fact is that when a sensor is immersed in a conductive fluid,energy from the sensor will dissipate, substantially lowering the Q andreducing the sensor-to-electronics distance. For example, the sensorsdescribed above were immersed in saline (0.9% salt solution), and themeasured Q decreased to approximately 10. It has been found that suchloss can be minimized by further separation of the sensor from theconductive liquid. This can be accomplished, for example, byencapsulating the sensor in a suitable low-loss-tangent dielectricmaterial. However, potential encapsulation material must have theflexibility and biocompatibility characteristics of the sensor materialand also be sufficiently compliant to allow transmission of fluidpressure to the pressure sensitive diaphragm. A preferred material forthis application is polydimethylsiloxane (silicone).

As an example, a thin (i.e., 200 micron) coating of silicone was appliedto the sensor detailed above. This coating provided sufficientinsulation to maintain the Q at 50 in a conductive medium. Equallyimportant, despite the presence of the silicone, adequate sensitivity topressure changes was maintained and the sensor retained sufficientflexibility to be folded for endovascular delivery. One additionalbenefit of the silicone encapsulation material is that it can beoptionally loaded with a low percentage (i.e., 10-20%) of radio-opaquematerial (e.g., barium sulfate) to provide visibility when examinedusing fluoroscopic x-ray equipment. This added barium sulfate will notaffect the mechanical and electrical properties of the silicone.

As described above, it is desirable to increase the Q factor of asensor, and the Q factor can be increased by suitable selection ofsensor materials or a coating, or both. Preferably both are used,because the resulting high Q factor of a sensor prepared in this fashionis especially suitable for the applications described.

When introduced into the sac of an abdominal aorta, the pressure sensorcan provide pressure related data by use of an external measuringdevice. As disclosed in the Allen et al. patents, several differentexcitation systems can be used. The readout device generateselectromagnetic energy that can penetrate through the body's tissues tothe sensor's implanted location. The sensor's electrical components canabsorb a fraction of the electromagnetic energy that is generated by thereadout device via inductive coupling. This coupling will induce acurrent in the sensor's circuit that will oscillate at the samefrequency as the applied electromagnetic energy. Due to the nature ofthe sensor's electromechanical system there will exist a frequency ofalternating current at which the absorption of energy from the readoutdevice is at a minimum. This frequency is a function of the capacitanceof the device. Therefore, if the sensor's capacitance changes so willthe frequency at which it minimally absorbs energy from the readoutdevice. Since the sensor's capacitance is mechanically linked to thefluid pressure at the sensor's surface, a measurement of this frequencyby the readout device can give a relative measurement of the fluidpressure. If calibration of the device is performed then an absolutemeasurement of pressure can be made.

The circuitry used to measure and display pressure is contained within asimple to operate, portable electronic unit 400, as shown in FIG. 28.This unit 400 also contains the antenna needed to perform theelectromagnetic coupling to the sensor. The antenna may be integratedinto the housing for the electronics or it may be detachable from theunit so that it can be positioned on the surface of the body 402 inproximity to the implanted sensor and easily moved to optimize thecoupling between antenna and sensor. The antenna itself may consist of asimple standard coil configuration or my incorporate ferrous elements tomaximize the coupling efficiency. The electronic device would feature anLCD or LED display 404 designed to clearly display the recorded pressurein physiologically relevant units such as mm Hg. In an alternativeembodiment, the display may be created by integrating a commerciallyavailable hand-held computing device such as a Palm® or micro-PC intothe electronic circuitry and using this device's display unit as thevisual interface between the equipment and its operator. A furtheradvantage of this approach is that the hand-held computer could bedetached from the read-out unit and linked to a standard desktopcomputer. The information from the device could thus be downloaded intoany of several commercially available data acquisition software programsfor more detailed analysis or for electronic transfer via hard media orthe internet to a remote location.

Accordingly, the present invention provides for an impedance system andmethod of determining the resonant frequency and bandwidth of a resonantcircuit within a particular sensor. The system includes a loop antenna,which is coupled to an impedance analyzer. The impedance analyzerapplies a constant voltage signal to the loop antenna scanning thefrequency across a predetermined spectrum. The current passing throughthe transmitting antenna experiences a peak at the resonant frequency ofthe sensor. The resonant frequency and bandwidth are thus determinedfrom this peak in the current.

The method of determining the resonant frequency and bandwidth using animpedance approach may include the steps of transmitting an excitationsignal using a transmitting antenna and electromagnetically coupling asensor having a resonant circuit to the transmitting antenna therebymodifying the impedance of the transmitting antenna. Next, the step ofmeasuring the change in impedance of the transmitting antenna isperformed, and finally, the resonant frequency and bandwidth of thesensor circuit are determined.

In addition, the present invention provides for a transmit and receivesystem and method for determining the resonant frequency and bandwidthof a resonant circuit within a particular sensor. According to thismethod, an excitation signal of white noise or predetermined multiplefrequencies is transmitted from a transmitting antenna, the sensor beingelectromagnetically coupled to the transmitting antenna. A current isinduced in the resonant circuit of the sensor as it absorbs energy fromthe transmitted excitation signal, the current oscillating at theresonant frequency of the resonant circuit. A receiving antenna, alsoelectromagnetically coupled to the transmitting antenna, receives theexcitation signal minus the energy which was absorbed by the sensor.Thus, the power of the received signal experiences a dip or notch at theresonant frequency of the sensor. The resonant frequency and bandwidthare determined from this notch in the power.

The transmit and receive method of determining the resonant frequencyand bandwidth of a sensor circuit includes the steps of transmitting amultiple frequency signal from transmitting antenna, and,electromagnetically coupling a resonant circuit on a sensor to thetransmitting antenna thereby inducing a current in the sensor circuit.Next, the step of receiving a modified transmitted signal due to theinduction of current in the sensor circuit is performed. Finally, thestep of determining the resonant frequency and bandwidth from thereceived signal is executed.

Yet another system and method for determining the resonant frequency andbandwidth of a resonant circuit within a particular sensor includes achirp interrogation system. This system provides for a transmittingantenna which is electromagnetically coupled to the resonant circuit ofthe sensor. An excitation signal of white noise or predeterminedmultiple frequencies, or a time-gated single frequency is applied to thetransmitting antenna for a predetermined period of time, therebyinducing a current in the resonant circuit of the sensor at the resonantfrequency. The system then listens for a return signal which is coupledback from the sensor. The resonant frequency and bandwidth of theresonant circuit are determined from the return signal.

The chirp interrogation method for determining the resonant frequencyand bandwidth of a resonant circuit within a particular sensor includesthe steps of transmitting a multi-frequency signal pulse from atransmitting antenna, electromagnetically coupling a resonant circuit ona sensor to the transmitting antenna thereby inducing a current in thesensor circuit, listening for and receiving a return signal radiatedfrom the sensor circuit, and determining the resonant frequency andbandwidth from the return signal.

The present invention also provides an analog system and method fordetermining the resonant frequency of a resonant circuit within aparticular sensor. The analog system comprises a transmitting antennacoupled as part of a tank circuit which in turn is coupled to anoscillator. A signal is generated which oscillates at a frequencydetermined by the electrical characteristics of the tank circuit. Thefrequency of this signal is further modified by the electromagneticcoupling of the resonant circuit of a sensor. This signal is applied toa frequency discriminator which in turn provides a signal from which theresonant frequency of the sensor circuit is determined.

The analog method for determining the resonant frequency and bandwidthof a resonant circuit within a particular sensor includes the steps ofgenerating a transmission signal using a tank circuit which includes atransmitting antenna, modifying the frequency of the transmission signalby electromagnetically coupling the resonant circuit of a sensor to thetransmitting antenna, and converting the modified transmission signalinto a standard signal for further application.

The invention further includes an alternative method of measuringpressure in which a non-linear element such as a diode orpolyvinylidenedifloride piezo-electric polymer is added to the LCcircuit. A diode with a low turn-on voltage such as a Schottky diode canbe fabricated using micro-machining techniques. The presence of thisnon-linear element in various configurations within the LC circuit canbe used to modulate the incoming signal from the receiving device andproduce different harmonics of the original signal. The read-outcircuitry can be tuned to receive the particular harmonic frequency thatis produced and use this signal to reconstruct the fundamental frequencyof the sensor. The advantage of this approach is two-fold; the incomingsignal can be transmitted continuously and since the return signal willbe at different signals, the return signal can also be receivedcontinuously.

The above methods lend themselves to the creation of small and simple tomanufacture hand-held electronic devices that can be used withoutcomplication.

The preceding specific embodiments are illustrative of the practice ofthe invention. It is to be understood, however, that other expedientsknown to those skilled in the art or disclosed herein, may be employedwithout departing from the spirit of the invention of the scope of theappended claims.

1. A sensor for wirelessly determining a physical property, which sensorcomprises: a self-contained resonant circuit comprising a capacitor andan inductor, and two substrates, at least one of which substrates has arecess therein, wherein the circuit is variable in response to thephysical property and wherein the substrates are hermetically sealedtogether.
 2. The sensor of claim 1, wherein the hermetically sealedsubstrates form a pressure sensitive chamber.
 3. The sensor of claim 1,wherein the substrates are comprised of glass, fused silica, sapphire,quartz, or silicone.
 4. The sensor of claim 3, wherein the substratesare comprised of fused silica.
 5. The sensor of claim 1, wherein thephysical property is pressure.
 6. The sensor of claim 1, wherein thereare no conductive connections or via holes to provide a direct physicalconduit or connection between an upper inductor coil and a lowerinductor coil.
 7. The sensor of claim 1, wherein each substrate has aninductor coil arranged therein in planar fashion.
 8. The sensor of claim7, wherein the inductor coil of one substrate is in a plane parallel tothe plane of the inductor coil in the second substrate.
 9. The sensor ofclaim 8, wherein the inductor coils are coextensive.
 10. The sensor ofclaim 1, wherein the inductor coil of the substrate with a recess ispositioned in the recess.
 11. The sensor of claim 10, wherein eachsubstrate has a recess and an inductor coil is positioned in eachrecess.
 12. The sensor of claim 1, wherein the inductor comprises toinductor coils and each inductor coil is a wire spiral.
 13. The sensorof claim 12, wherein each wire spiral is formed by electrodeposition.14. The sensor of claim 1, wherein the sensor is from about 0.5 in. toabout 1 in. in length and from about 0.1 in. to about 0.5 in. in width.15. The sensor of claim 14, wherein the sensor has a thickness of fromabout 0.05 in. to about 0.30 in.
 16. The sensor of claim 1, wherein astabilizer is arranged around the sensor.
 17. The sensor of claim 16,wherein the stabilizer stabilizes position, location, and/ororientation.
 18. The sensor of claim 16, wherein the stabilizer is ametal basket arranged around the outer surface of the sensor.
 19. Thesensor of claim 1, wherein the physical property is measured in apatient.
 20. A system for delivering a sensor, which comprises a sensorof claim 1 and a delivery guidewire or catheter.
 21. The system of claim21, wherein the sensor is removable from the guidewire or catheter. 22.The system of claim 21, wherein the sensor is removably attached to theguidewire or catheter.
 23. The system of claim 21 which also comprisesan instrument for measuring signals from the sensor.
 24. A method formeasuring a physical property which comprises the steps of inserting asensor of claim 1 into a desired location and then measuring any signalsgenerated by said sensor.
 25. The method of claim 24, wherein a physicalproperty in a patient is measured.